The present invention relates generally to an infrared-detector array including a plurality of infrared detectors arranged in a matrix pattern. More specifically, this invention relates to a thermal infrared-detector array which detects infrared radiation from objects based on change in temperature-dependent characteristic of the detector caused by the infrared radiation absorption.
An infrared-detector array can not only take pictures of an object invisible to human eyes, but also measure a temperature of a distant object on a non-contact basis. The infrared-detector array is used for a variety of industrial and/or medical purposes such as, for example, measuring and controlling a fabrication line, diagnosing a human body, or detecting a human.
The infrared-detector array comprises a plurality of infrared detectors arranged in a matrix pattern and a signal output circuit disposed in the vicinity thereof for extracting a signal from the detector array.
The infrared detector is broadly available in two types depending on the basic mechanism of operation: One is a xe2x80x9cquantum-typexe2x80x9d capable of measuring an infrared radiation as photons, and the other is a xe2x80x9cthermal typexe2x80x9d capable of measuring the infrared radiation as an change in temperature change caused by the absorption of the infrared radiation. Although the xe2x80x9cquantum typexe2x80x9d detector is highly sensitive and has a short response time, it has a complicated structure and is highly expensive because it must be refrigerated to about xe2x88x92200xc2x0 C. On the other hand, the xe2x80x9cthermalxe2x80x9d detector is simple in structure and can operate at ambient temperature even though the response time is relatively slow and, thus, it is widely used for general purposes.
The infrared-detector array comprising a plurality of bolometer-type detectors, which are disclosed in, for example, U.S. Pat. No. 5,260,255, is commercially available. The bolometer-type detector measures the infrared radiation in terms of change in resistance caused by the absorption of infrared radiation from an object.
FIG. 7 shows a perspective illustration of the bolometer-type infrared detector comprising a support substrate 804, an insulating layer 803, leads 802 and resistance strips 801 supported above the insulating layer 803 by means of a plurality of, for example, diagonally opposite legs 801a that are formed by the use of a micromachining technique. When each of the resistance strips 801 are heated by infrared radiation, the resistance of the resistance strip 801 undergoes a change, and the change in resistance of the resistance strip 801 can be observed in terms of change in bias current or voltage applied to the resistance strip 801 through the leads 802 The resistance strip 801 is in the form of a thin film made of, for example, metal, ceramics such as vanadium oxide, or a polycrystal silicon. However, these materials for the resistance strip tend to bring about some problems associated with mass-productivity and performance.
By way of example, where the metallic thin film is used as a resistance strip for the detector, the rate of temperature dependent change of the resistance which is hereinafter referred to as a xe2x80x9cTemperature Coefficient of Resistance, (or TCR for short)xe2x80x9d is so low, for example, about 0.5%/K, that the sensitivity of the detector as a whole to the infrared radiation is short of the required sensitivity. Vanadium oxide has a relatively high TCR value, about 2.0%/K. However, the vanadium oxide is not used in the semiconductor-fabrication process because it may contaminate other semiconductor elements. Therefore, the infrared-detector array using vanadium oxide as a material for the resistance strip cannot be fabricated together with the signal output circuit and poses a problem that the detectors must be formed in a process separate from a process of making the signal output circuit. If the resistance material is employed in the form of a polycrystal silicon, the infrared detector can be integrated with the signal-output circuits. However, the S/N ratio of the infrared detector using the polycrystal silicon as a material for the resistance strip is poor because the doped impurity concentration in polycrystal silicon must be decreased to keep the TCR so high as to render the detector to be highly sensitive. If the doped impurity concentration is low, conduction by a trap level formed between grains in polycrystal silicon becomes dominant and, thus, the S/N ratio of the infrared detector becomes poor.
The thermal infrared detector utilizing a semiconductor-junction element has been suggested in, for example, the Japanese Laid-open Patent Publications No. 9-166497 and No. 8-186283. The semiconductor-junction element is, for example, a p-n junction formed on monocrystalline silicon; Schottky barrier diode; or other kind of transistor.
The infrared detector using the semiconductor-junction element detects the infrared radiation by utilizing a current-voltage characteristic change caused by change in temperature. In general, the semiconductor layers in the semiconductor-junction element has a high impurity concentration and a high crystallinity. Accordingly, the infrared detector using the semiconductor-junction element has a low resistance noise and a high S/N ratio.
However, with semiconductor-junction element formed directly on a monocrystalline silicon substrate, no efficient temperature increase by infrared absorption occurs in the semiconductor junction element, with the detector as a whole failing to provide a sufficient sensitivity because of a high thermal conductivity of the silicon substrate. The Japanese Laid Open Patent Publication No. 8-186283 describes that an SOI structure may be applied to the infrared detector to improve the thermal sensitivity. In the SOI structure, the semiconductor-junction elements are isolated from the silicon substrate by a silicon oxide having a low thermal conductivity.
The infrared detector using semiconductor-junction elements has advantages in that it can be fabricated in a semiconductor-fabrication process and in that it has a high sensitivity and a low noise. However, no infrared-detector array has been made available, wherein the semiconductor-junction elements are arranged in a matrix pattern and are integrated with the signal-output circuit.
An objective of the present invention is to provide an infrared-detector array including semiconductor-junction elements as infrared detectors and a method of fabrication thereof.
In accomplishing the above and other objectives, one aspect of the present invention provides an infrared-detector array which comprises a plurality of thermal-type infrared detectors arranged in linear or matrix, and a signal-output circuit reading out signals from the infrared detectors;
wherein the infrared detectors include semiconductor-junction elements formed in a monocrystalline-silicon layer overlying a silicon-oxide layer on a monocrystalline-silicon substrate, and wherein the signal-output circuit includes transistors formed on the monocrystalline-silicon substrate.
The invention increases the breakdown voltage of the transistors in the signal-output circuit, and driving voltage of the detectors. Thus the sensitivity of the infrared-detector array is increased.
Preferably, the monocrystalline-silicon substrate is partly removed to form cavities under the semiconductor-junction elements, so that the semiconductor-junction elements are thermally insulated.
The semiconductor-junction elements are preferably diodes, bipolar transistors, junction field-effect transistors, or MOS transistors.
In another aspect of the present invention, a process of producing the infrared-detector array comprises the steps of:
(A) preparing a substrate wherein a monocrystalline-silicon layer is overlying a silicon-oxide layer on a monocrystalline-silicon substrate;
(B) removing the monocrystalline-silicon layer in a transistor-on-substrate region where transistors of the signal-output circuit are formed directly on the monocrystalline-silicon substrate;
(C) removing the silicon-oxide layer in the transistor-on-substrate region;
(D) forming the semiconductor-junction elements in the monocrystalline-silicon layer, and at least a part of transistors of the signal-output circuit directly on the monocrystalline-silicon substrate.
In step (B), it is preferable to etch the monocrystalline-silicon layer to form islands in a region where the semiconductor-junction elements are formed in the monocrystalline-silicon layer.
In still another aspect of the invention, a process of producing an infrared-detector array comprises the steps of:
(A) preparing a substrate wherein a monocrystalline-silicon layer is overlying a silicon-oxide layer on a monocrystalline-silicon substrate;
(B) oxidizing by LOCOS (Local Oxidation of Silicon) method the monocrystalline-silicon layer all-over a transistor-on-substrate region where transistors of the signal-output circuit are formed directly on the monocrystalline-silicon substrate; and partly, leaving appropriate spaces, in a element-forming region where the semiconductor-junction elements are formed in the monocrystalline-silicon layer;
(C) removing the oxidized monocrystalline-silicon layer and the silicon-oxide layer in the transistor-on-silicon region;
(D) forming the semiconductor-junction elements in the monocrystalline-silicon layer, and at least a part of transistors of the signal-output circuit directly on the monocrystalline-silicon substrate.
According to this process of the present invention, a step at the end of the monocrystalline-silicon layer becomes smooth. As a result, fabrication problems, such as etching residue and lead disconnection, are suppressed.